The present invention relates to a multiscreen display apparatus for forming one screen by combining a plurality of image display units, and in particular to an apparatus for correcting luminance shading and color shading of the multiscreen display apparatus.
In multiscreen displays, one large screen is formed by piling up a plurality of display units 1a-1d as shown in FIG. 1. In case the same picture area is to be obtained, such multiscreen displays can be shortened in depth of entire apparatus and have comparatively high luminance as compared with single large-picture displays of front projection type and back projection type. Therefore, such multiscreen displays are used in event sites and showrooms, for example. As individual display units (hereafter referred to as cores), display units of cathode-ray tube (CRT) direct-view type or projection type have been put to practical use. Above all, cores of projection type have been put to frequent use because they are lighter in weight and have flat faces as compared with cores of direct-view type. However, cores of CRT direct-view type and projection type have inherent luminance shading in which the peripheral part (shaded region of FIG. 1) is typically darker than the central region of the screen. Especially in case of multiscreen display, that shading becomes prominent. In a measure against such a problem as described in JP-A-57-111187, overscan is so performed as to cause overlap to compensate for the luminance lowering in the peripheral parts between cores and thereby make the screen luminance uniform.
In the technique disclosed in the above described JP-A-57-111187, however, images must be aligned in overscan regions between cores and all characteristics such as convergence and distortion correction must be matched between cores. This technique has thus many problems to be solved for practical use. Therefore, nonuniformity in luminance should be solved for each core.
FIG. 2 is an example of a block diagram showing the configuration of a multiscreen display apparatus. Numeral 40 denotes an image expansion device, 4a-4d and 41 video signal input terminals, 1a-1d cores, 6 a comparison circuit for ABL, and 60a-60d ABL control information input terminals. A video signal output of the image expansion device 40 is so converted on time axis and applied to the upper left core 1a that the former halves of a horizontal scanning interval and a vertical scanning interval of an ordinary video signal may be so displayed as to fill up the screen. In the same way, the video signal output of the image expansion device 40 is so converted on time axis and applied to the upper right core 1b that the latter half of a horizontal scanning interval of the above described video signal and the former half of a vertical scanning interval of the video signal may be so displayed as to fill up the screen. With respect to the cores 1c and 1d as well, similar time-axis conversion processing is applied.
In this case, contents of video signals inputted to respective cores basically differ. If ABL (automatic brightness limit) functions independently in respective cores, therefore, contrasts of respective cores differ. As shown in FIG. 2, therefore, respective cores output ABL control information to the input terminals 60a-60d, respectively. Further, the display apparatus is so configured that the ABL control information of a core having the highest average luminance may be detected by the comparison circuit 6 for ABL and screen luminance of all cores may be controlled in common in accordance with the ABL control information thus detected. As a result, ABL does not function independently for respective cores, but in principle control is so exercised that the luminance of all cores may become constant.
In the core outputting the maximum value of the ABL control information, its own control information is fed back and hence closed loop control using ABL is effected. In each of remaining cores, however, control information transmitted from other cores is supplied instead of its control information, resulting in open loop control. Because of dispersion of gains of respective open loops, i.e., because of dispersion of gains of the contrast and/or luminance control circuit and an ABL circuit, contrasts of cores do not perfectly coincide with each other, resulting in a problem of dispersion of luminance level. For example, it is now assumed that gray having a constant luminance is displayed on the four cores of FIG. 1 and a character or the like having a high luminance is added to only the core 1a. If the average luminance of the screen 1a is at least a predetermined luminance, the ABL automatically outputs the ABL control information to the contrast and/or luminance control circuit to lower the average luminance of the screen of 1a. The average luminance of other cores 1b-1d is also lowered by the ABL control information of the above described 1a. If gains of the contrast and/or luminance control circuit and the ABL circuit have dispersion, however, there occurs difference between luminance levels of 1a-1d, resulting in a problem of discontinuity of luminance at boundary between cores.